It was an event the whole world was watching, but some deaf people couldn't understand a word spoken at Mandela’s memorial and took to Twitter to vent their frustration.

Allegations have been made that the sign language interpreter used at the memorial may have been unqualified.

Braam Jordaan, a deaf South African citizen and board member of the World Federation of the Deaf Youth Section (WFD), told SBS he believes the interpreter is a fake.

"The structure of his hand, facial expressions and the body movements did not follow what the speaker was saying," he told SBS via email.

Mr Jordaan says the interpreter, who was signing for a portion of the ceremony including Obama's speech, was simply making up his own signs.

"I was really upset and humiliated by the mystery interpreter who was supposed to be signing what Barack Obama, the president of United States of America was saying," he says. "He made up his own signs."

"What happened at the memorial service is truly disgraceful thing to see - it should not happen at all.

Mr Jordaan says there have been question marks over the same interpretor at previous ANC events.

Watch: The interpreter signing along to Jacob Zuma at ANC Centenery Celebrations in Bloemfontein, South Africa in January 2012.

Mr Jordaan told SBS that if the interpeter is proven to be unqualified, this could be a breach of the United Nations Convention on the Rights of Persons with Disabilities, which was ratified by the South African government in 2007.

Article 21 of the Convention says that 'States Parties' should take 'appropriate measures' to provide professional sign language interpreters.

The Oceania representative for the World Association of Sign Language Interpreters (WASLI), Sheena Walters, also states that the signing used was not recognisable as any accepted form of international or South African sign language.

“It seems quite obvious that the interpreter isn’t using South African sign language,” she told SBS.

“Most sign languages across the world share a similar structure and pattern and this person seems to be making a lot of repetitive signs and isn’t displaying the usual facial expression or structure of sign language that you would normally see.”

Some members of the deaf community took to Twitter to express outrage.

Wilma Newhoudt, a deaf member of the South African Parliament and Vice President of the World Federation of the Deaf, labelled the signing as "shameful".

Shame on this male so called interpreter on the stage. What is he signing? He knows that the deaf cannot vocally boo him off. Shame on him !